Project Pluto

In the 1950s and early 1960s, the development of controlled release
of nuclear energy (as opposed to runaway reactions in nuclear
weapons) seemed to make previously impossible things
straightforward matters of engineering. On occasion, it made the
unthinkable all too thinkable. Project
Pluto, also known as the Supersonic
Low Altitude Missile (SLAM), is one of the most extreme
exemplars of this epoch. Unlike Project
Orion, which envisioned a four thousand ton interplanetary
spaceship powered by nuclear bombs, and Project
Plowshare, which proposed using nuclear explosions for large
civil engineering projects such as digging a new sea-level Panama
canal, Pluto/SLAM (which I'll henceforth refer to as Pluto) was
developed and tested to the point where the technology had been
proved. Pluto could have been deployed in the 1960s had the project
not been cancelled due to financial reasons, competition from rival
technologies, and people coming to their senses.

Pluto was a nuclear powered ramjet cruise missile, as large
and heavy as a steam locomotive, which could fly between three and
four times the speed of sound either at high altitudes or skimming
the earth, with global range and the ability to stay airborne for
months, and then to rain nuclear destruction upon multiple
targets with high accuracy. Let's unpack this description to
understand just was was being developed.

The key distinction between nuclear and chemical
energy (whether in explosives, propulsion, or generation of heat)
is that while a typical chemical reaction (for example, combustion
of hydrogen and oxygen to yield water) releases energy on the order
of a few electron
volts per molecule, a nuclear reaction, such as the fission of
uranium, releases a few million electron volts per atom.
Thus, a given mass of nuclear fuel yields around a million times as
much energy as the most energetic chemical reactions. All of the
capabilities of nuclear energy, for good or ill, derive from this.

In the 1950s, the United States developed three principal means of
delivering strategic nuclear weapons: the so-called
strategic triad which remains in place today. First
to enter service was the manned bomber. Bombers
offered precision strike capability and could be recalled at any
time, but they took a long time to reach targets on other
continents and were vulnerable to air defenses, both manned
interceptors and missiles. Keeping a bomber force on alert was
expensive, as multiple crews had to be trained and kept proficient,
and the bomber hardware required regular maintenance.
Guided missiles, the second leg of the triad,
provided an alternative to the bomber force. First to be developed
were cruise
missiles, pilotless aircraft equipped with a guidance system to
direct them to their targets where a warhead will be detonated.
Much smaller than manned bombers, cruise missiles are less
expensive, and can be kept in a state of readiness without training
flights. Early cruise missiles were no faster than manned bombers,
but as the fifties progressed, supersonic cruise missiles were
developed. Ballistic
missiles are rockets which lob a warhead directly to the
target. An intercontinental ballistic missile can hit targets on
other continents within thirty minutes of launch, and be based
in hardened shelters which make them hard to destroy
in an enemy first strike. The third leg involved
missiles launched from submarines rather than
bases on land. Since it is very difficult to detect and track a
submerged submarine, these missiles are essentially invulnerable to
a first strike, and provide an assured ability to retaliate.
However, at the time, submarine-launched missiles had neither the
long range nor the accuracy of those based on land.

With any strategic weapon, one worries about a technological leap
by the adversary which will render one's own weapons obsolete. For
those whose job was to worry about such things, there were
numerous causes for concern. It takes many years to develop
and field a new manned bomber, and there's always the risk that
progress in air defenses will make it impossible for the bombers to
reach their targets. Research into anti-ballistic missile defenses
was an active topic in the 1950s and '60s, and there were several
promising approaches which might blunt the effectiveness of both
land- and sea-based missiles. Finally, detection of submarines was
a cat-and-mouse game where both sides were investing in
technologies to counter the threat. Success would negate the
invulnerability of the submarine-launched deterrent force.

A major justification for the strategic triad was that should an
adversary field a defense which countered the effectiveness of one
leg of the triad, the remaining legs would continue to provide
deterrence until a response was fielded. Maintaining a diverse
portfolio of weapon delivery systems avoided
betting everything on the effectiveness of one technology. It
isn't cheap, the argument went, but it's a lot cheaper than the
aftermath of an enemy first strike invited by a perceived
vulnerability.

It was into this environment that Project Pluto was proposed. Its
unique capabilities made it almost qualify as an additional
leg of deterrence, and one which would be extraordinarily difficult
to counter. Central to this was its power plant: a nuclear
ramjet. A conventional turbojet engine
is shown below.

Air enters the engine at the left, and is compressed by a series of
spinning compressor discs. The air then enters combustion chambers
where it and fuel are burned, producing hot gases under pressure.
These gases pass through a number of turbine discs, which
extract sufficient energy to spin the compressor. The hot gases are
then expelled through the exhaust nozzle, producing thrust
according to Newton's
third law of motion. The compressor is required to achieve
the high energy combustion necessary to produce thrust.

But if the engine is moving rapidly enough through the air, its own
motion can compress the air entering it sufficiently, eliminating
the need for a compressor and the turbine that powers it. This is
called a ramjet, as it is
the force of air being rammed into the engine which produces the
required compression.

As is evident, this is a dramatic simplification over the turbojet.
The only tricky part is designing the engine inlet to obtain the
required compression of the incoming air. There is, however, a
catch. A ramjet can only achieve the compression it needs to
operate when moving at supersonic speeds: ideally around three
times the speed of sound (Mach 3). Consequently, a ramjet requires
a booster (usually a rocket) to accelerate it to operating speed,
after which it can operate in a steady state. Once the air has been
compressed by the inlet, fuel is injected and burned, which
produces the hot gases expelled by the nozzle. Once a ramjet begins
to operate, it will continue to produce thrust as long as fuel is
supplied to burn with the incoming air. Ramjets are much more fuel
efficient than rockets, since they do not need to carry their own
oxidiser, but instead use oxygen from the air. Still,
they tend to be “thirsty”, and the amount of fuel which can be
carried limits their range.

But the only function of burning the fuel in a ramjet is to
generate hot gas under high pressure to expel from the nozzle,
yielding thrust. Suppose we replaced the combustors in the engine
with a nuclear reactor. Air, passing through the reactor, would be
heated to its operating temperature and expelled through the nozzle
as before. No combustion takes place; it's cold air in and hot air
out. The only limit to endurance is the lifetime of the nuclear
fuel (measured in years) and the ability of the airframe and other
components to withstand the stresses of flight. While combustion is
sensitive to air pressure and temperature, with no combustion in
the engine, the nuclear ramjet could operate equally well
at low altitude and high, and provide performance “on the
deck” unmatched by any other kind of engine.

The potential of the nuclear ramjet was such that studies of both
engines and missiles began in the mid-1950s. In 1957, the Lawrence
Radiation Laboratory (later Lawrence Livermore National Laboratory)
began detailed design studies of ramjet propulsion reactors. The
flight reactor was intended to have thermal power in excess of
500 megawatts, but to prove the concept a sub-scale reactor was
built first. This reactor, designated Tory II-A, had a design power
of 155 megawatts. It heated incoming air to a temperature of
1080 °C, and had a flow rate of 320 kg/sec. As a non-flying
test bed, no effort was made to reduce the weight of this reactor.

Recall that a ramjet only works when the air entering it is moving
faster than sound. Further, compression of the air by the engine's
inlet would heat it to around 540 °C. To test the engine under
realistic conditions, an enormous tank farm was built at the test
site at Jackass Flats, Nevada, using oil well casing pipe to store
54,000 kg of air compressed to 245 atmospheres. Before air entered
the engine, it was heated by passing it through a vessel containing
544,000 kg of ball bearings heated by a gas furnace. This allowed
running the engine at full power for up to one minute.

Several tests were run in 1961, and other than some minor cracking
in the fuel elements, the reactor performed to specifications. Once
the reactor had gone critical for the first time, it was extremely
radioactive and, having no shielding, would have been lethal to
anybody who approached it, even when it was not running. The
reactor was installed on a railroad car, which an automated
locomotive could transfer between the test stand and the
assembly/disassembly building, where heavily shielded test cells
allowed it to be manipulated by remote control. With these
procedures in place, there seemed no obstacle to developing the
full scale engine.

With the success of Tory II-A, work began on Tory II-C, a
full-scale, flight-weight reactor capable of sustained low altitude
flight in excess of Mach 3. With design power of 500 megawatts and
much greater airflow, the tank farm had to be expanded by a
factor of ten, employing 40 km (25 miles) of oil well casing pipe,
which took five days to fill with air. This sufficed for a five
minute test run of Tory II-C, which was sufficient to demonstrate
steady-state operation and measure the thermal and radiation
environment around the operating reactor. While further
improvements to the design were on the drawing board, in two test
runs in 1964 Tory II-C demonstrated that it would get the job done.

With the engine development proceeding toward flight hardware, in
1963 Ling-Temco-Vought was awarded the contract to develop the
airframe, with Marquardt chosen as contractor for propulsion
hardware other than the reactor. Intended to fly near Mach 3 in the
thick lower atmosphere, the missile would undergo aerodynamic and
thermal stresses never before encountered. With the nuclear
powerplant, however, there was no need to trade off weight and
strength as in conventional aircraft. Most of the structure was
made of stainless steel, with high temperature sections near the
reactor using the René 41 alloy earlier used in the Mercury
spacecraft. So robust was the structure that project director Ted
Merkle nicknamed it “the flying crowbar”. With no need to
carry fuel on board, the design was simple: nuclear engine,
guidance system, and warhead(s), with small control surfaces to
steer. There were no wings; at supersonic speeds, the shape of the
missile body provided sufficient lift. Solid rocket boosters would
lift Pluto from its launcher (both fixed and mobile launchers
were envisioned) and accelerate it to the speed where the ramjet
could operate. The reactor would not go critical until the moment
of launch, rendering it safe for ground personnel in pre-launch
operations.

Pluto/SLAM mission profile. Ling-Temco-Vought
illustration.

Armament is variously described in contemporary documents, and one
draws the conclusion that a firm decision was never made as to the
weapons load operational missiles would carry. Most sources
envision an option of a single very high yield bomb or multiple
(anywhere from 8 to more than 20) smaller bombs. In the multiple
bomb configuration, the bombs would be delivered to separate
targets as the missile followed its programmed flight path. With
unlimited range and endurance, there were no limits on the attack
profile. The guidance system would have been extremely accurate.
After using inertial
navigation to fly at high altitude to the border of enemy
territory, Pluto would dive to treetop level and begin its dash to
the target(s), guided by a radar system which compared terrain
passing under the missile with a stored terrain model. This was
expected to deliver weapons on target with an error on the order of
tens of metres. Modern cruise missiles use a system called TERCOM, which is a
direct derivative of this design.

Unlike naval propulsion or civil nuclear power reactors, Pluto's
reactor had no radiation shielding whatsoever. Avionics would be
placed as far from the reactor as possible and in a shielded vault
to protect it from the intense neutron flux. The radiation emitted
from the operating reactor would be lethal to anybody in the
vicinity, including people beneath its flight path when operating
at low altitude. In addition, the shock wave from low level
supersonic flight would destroy unreinforced structures and
severely injure people who weren't already killed by the radiation.
A Strangelovian suggestion was that the missile, after releasing
its last bomb, could simply fly around enemy territory, destroying
structures and killing people with its radiation. Even when in high
altitude flight en route to the target country, the engine would
spew fission fragments in its exhaust, leaving a trail
of fallout behind it. Critics said the acronym SLAM should
really stand for “Slow, Low, and Messy”. Nobody ever came up
with a satisfactory way to flight test the missile. It was
suggested that test flights might be conducted over the Pacific,
with the missile ditched into the deep ocean at the end of the
flight, but even in the 1960s, this was likely to face public
opposition. Finally, strategists feared that deploying a new weapon
against which there was no existing defense would destabilise
the strategic balance and provoke the Soviets into developing their
own version. Adding a Soviet
Плутоский
to the existing nuclear
threat wasn't a prospect anybody welcomed.

Through 1963, total funding for Project Pluto, including its test
facilities, was US$ 260 million in then dollars,
or 2 billion today. Estimated unit cost for a purchase of
fifty operational missiles (less warheads) was around US$ 50
million (400 million today). With the McNamara whiz kids in the
Pentagon on the warpath to cut budgets, and with Pluto not
estimated to be operational before 1969, it was an irresistible
target, and in July 1964 the project was cancelled. By then,
ballistic missiles were operational, and the solid fuel Minuteman
missile, which was flight-proven and cost a fraction of Pluto's
price, was entering mass production. Pluto was the solution to a
problem which no longer existed, at a price which wasn't
affordable.

Project Pluto was the last episode in the grand romantic era of
nuclear power in the postwar period. In the 1950s, nuclear
generated electric power “too cheap to meter”, nuclear
automobiles, and nuclear airplanes seemed right around the
corner. In the New York Times of June 10, 1955, Alex
Lewyt, president of the Lewyt Vacuum Cleaner Company, was quoted
predicting “Nuclear powered vacuum cleaners will probably be a
reality within 10 years”. In fact, it was nuclear reality that had
set in, and the pendulum would soon swing to an irrational fear of
a technology which, intelligently applied where appropriate,
still retains its million-to-one advantage over chemical
energy that inspired the optimism of the early atomic age.

Project Pluto is a documentary about the project,
including contemporary film of the development and test phases. The
film is presented in five parts in a play
list, which you can view below.

Big Stick was a competitive design for a nuclear ramjet missile
proposed by the Convair division of General Dynamics. Here is a 1959
film describing the concept.